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Resim Yükleyin
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Ýlgili Makaleler
High-contrast Imaging Search for Planets and Brown Dwarfs around the Most Massive Stars in the Solar Neighborhood
There has been a long-standing discussion in the literature as towhether core accretion or disk instability is the dominant mode ofplanet formation. Over the last decade, several lines of evidence havebeen presented showing that core accretion is most likely the dominantmechanism for the close-in population of planets probed by radialvelocity and transits. However, this does not by itself prove that coreaccretion is the dominant mode for the total planet population, sincedisk instability might conceivably produce and retain large numbers ofplanets in the far-out regions of the disk. If this is a relevantscenario, then the outer massive disks of B-stars should be among thebest places for massive planets and brown dwarfs to form and reside. Inthis study, we present high-contrast imaging of 18 nearby massive starsof which 15 are in the B2-A0 spectral-type range and provide excellentsensitivity to wide companions. By comparing our sensitivities to modelpredictions of disk instability based on physical criteria forfragmentation and cooling, and using Monte Carlo simulations for orbitaldistributions, we find that ~85% of such companions should have beendetected in our images on average. Given this high degree ofcompleteness, stringent statistical limits can be set from thenull-detection result, even with the limited sample size. We find that<30% of massive stars form and retain disk instability planets, browndwarfs, and very low mass stars of <100 M jup within 300AU, at 99% confidence. These results, combined with previous findings inthe literature, lead to the conclusion that core accretion is likely thedominant mode of planet formation.Based on data from the Gemini North telescope under programsGN-2008B-Q-59, GN-2009A-DD-6, GN-2010A-Q-86, and GN-2010B-Q-14, on Keckdata under program A200N2, and on Subaru data under program S09B-016.
| The inverse problem of the theory of degenerate dwarfs
Based on the radii and masses of degenerate dwarfs derived fromHIPPARCOS and other observations, we estimate the microscopic parametersof a Chandrasekhar model (the relativistic parameter at the stellarcenter x 0, and the chemical-composition parameterµ e = A/Z, where A is the mass number and Z is thenuclear charge). We have obtained analytical expressions for themacroscopic characteristics (mass, radius, energy) as functions of x0 and µ e . From the calculated dependenceof the energy on these parameters, we have found constraints on therange of variability of x 0, which are in good agreement withthe observed radius distribution of dwarfs. The critical value of x0 at which stability breaks down due to general-relativisitceffects is found more accurately than previously. We propose ageneralized model with an inhomogeneous (coordinate-dependent) chemicalcomposition, with µ e = µ e ( r).
| The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopicsubtypes, and stellar population subcomponents of the white dwarfswithin 20 pc of the Sun. We find no convincing evidence of halo whitedwarfs in the total 20 pc sample of 129 white dwarfs nor is thereconvincing evidence of genuine thick disk subcomponent members within 20parsecs. Virtually, the entire 20 pc sample likely belongs to the thindisk. The total DA to non-DA ratio of the 20 pc sample is 1.6, amanifestation of deepening envelope convection which transforms DA starswith sufficiently thin H surface layers into non-DAs. The addition offive new stars to the 20 pc sample yields a revised local space densityof white dwarfs of (4.9 ± 0.5) × 10–3pc–3 and a corresponding mass density of (3.3 ±0.3) × 10–3 M sunpc–3. We find that at least 15% of the white dwarfswithin 20 parsecs of the Sun (the DAZ and DZ stars) have photosphericmetals that possibly originate from accretion of circumstellar material(debris disks) around them. If this interpretation is correct, thissuggests the possibility that the same percentage have planets orasteroid-like bodies orbiting them.
| On the evolutionary status of short-period cataclysmic variables
We present high-speed, three-colour photometry of seven short-period(Porb <= 95 min) eclipsing cataclysmic variables (CVs)from the Sloan Digital Sky Survey. We determine the system parametersvia a parametrized model of the eclipse fitted to the observed lightcurve by ?2 minimization. Three out of seven of thesystems possess brown dwarf donor stars and are believed to have evolvedpast the orbital period minimum. This is in line with the predictionsthat 40-70 per cent of CVs should have evolved past the orbital periodminimum. Therefore, the main result of our study is that the missingpopulation of post-period minimum CVs has finally been identified. Thedonor star masses and radii are, however, inconsistent with modelpredictions; the donor stars are approximately 10 per cent larger thanexpected across the mass range studied here. One explanation for thediscrepancy is the enhanced angular momentum loss (e.g. fromcircumbinary discs); however, the mass-transfer rates, as deduced fromwhite dwarf effective temperatures, are not consistent with enhancedangular momentum loss. We show that it is possible to explain the largedonor radii without invoking enhanced angular momentum loss by acombination of geometrical deformation and the effects of starspots dueto strong rotation and expected magnetic activity. Choosingunambiguously between these different solutions will require independentestimates of the mass-transfer rates in short-period CVs.The white dwarfs in our sample show a strong tendency towards highmasses. We show that this is unlikely to be due to selection effects.The dominance of high-mass white dwarfs in our sample implies thaterosion of the white dwarf during nova outbursts must be negligible, oreven that white dwarfs grow in mass through the nova cycle. Amongst oursample, there are no helium-core white dwarfs, despite predictions that30-80 per cent of short-period CVs should contain helium-core whitedwarfs. We are unable to rule out selection effects as the cause of thisdiscrepancy.
| A Far Ultraviolet Archival Study of Cataclysmic Variables. I. FUSE and HST STIS Spectra of the Exposed White Dwarf in Dwarf Nova Systems
We present a synthetic spectral analysis of Far UltravioletSpectroscopic Explorer (FUSE) and Hubble Space Telescope Space TelescopeImaging Spectrograph (HST STIS) spectra of five dwarf novae above andbelow the period gap during quiescence. We use our synthetic spectralcode, including options for the treatment of the hydrogenquasi-molecular satellite lines (for low-temperature stellaratmospheres), non-LTE (NLTE) approximation (for high-temperature stellaratmospheres), and for one system (RU Peg) we model the interstellarmedium (ISM) molecular and atomic hydrogen lines. In all the systemspresented here the FUV flux continuum is due to the white dwarf (WD).These spectra also exhibit some broad emission lines. In this work weconfirm some of the previous FUV analysis results, but we also presentnew results. For four systems we combine the FUSE and STIS spectra tocover a larger wavelength range and to improve the spectral fit. Thiswork is part of our broader HST archival research program, in which weaim to provide accurate system parameters for cataclysmic variablesabove and below the period gap by combining FUSE and HST FUV spectra.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| a New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of whitedwarfs lying within 20 pc of the Sun. A new revised catalog of localwhite dwarfs containing 122 entries (126 individual degenerate stars) ispresented. This list contains 27 white dwarfs not included in a previouslist from 2002, as well as new and recently published trigonometricparallaxes. In several cases new members of the local white dwarfpopulation have come to light through accurate photometric distanceestimates. In addition, a suspected new double degenerate system (WD0423+120) has been identified. The 20 pc sample is currently estimatedto be 80% complete. Using a variety of recent spectroscopic,photometric, and trigonometric distance determinations, we re-compute aspace density of 4.8 ± 0.5 × 10 3 pc3 corresponding to a mass density of 3.2 ± 0.3 ×10 3 M sun pc 3 from the completeportion of the sample within 13 pc. We find an overall mean mass for thelocal white dwarfs of 0.665 M sun, a value larger than mostother non-volume-limited estimates. Although the sample is small, wefind no evidence of a correlation between mass and temperature in whichwhite dwarfs below 13,000 K are systematically more massive than thoseabove this temperature. Within 20 pc 25% of the white dwarfs are inbinary systems (including double degenerate systems). Approximately 6%are double degenerates and 6.5% are Sirius-like systems. The fraction ofmagnetic white dwarfs in the local population is found to be 13%.
| WD0433+270: an old Hyades stream member or an Fe-core white dwarf?
Context: G39-27/289 is a common proper-motion pair formed by a whitedwarf (WD0433+270) and a main-sequence star (BD+26 730) that apparentlyhas been classified as a member of the Hyades open cluster. Previousstudies of the white dwarf component yielded a cooling time of ~4 Gyr.Although it has not been pointed out before explicitly, this result is 6times greater than the age of the Hyades cluster, giving rise to anapparent conflict between the physics of white dwarfs and clustermain-sequence fitting. Aims: We investigate whether this systembelongs to the Hyades cluster and, accordingly, give a plausibleexplanation for the nature of the white dwarf member. Methods: Wehave performed and analyzed spectroscopic observations to bettercharacterize these objects, and used their kinematic properties toevaluate their membership in the Hyades. Then, different mass-radiusrelations and cooling sequences for different core compositions (He,C/O, O/Ne and Fe) were employed to infer the mass and cooling time ofthe white dwarf. Results: From kinematic and chemical compositionconsiderations, we believe that the system was a former member of theHyades cluster and therefore has an evolutionary link with it. However,the evidence is not conclusive. With regards to the nature of the whitedwarf component, we find that two core compositions - C/O and Fe - arecompatible with the observed effective temperature and radius. Thesecompositions yield very different cooling times of ~4 Gyr and ~1 Gyr,respectively. Conclusions: We distinguish two possible scenarios.If the pair does not belong to the Hyades cluster but only to the Hyadesstream, this would indicate that such a stream contains rather oldobjects and is definitely not coeval with the cluster. This hasinteresting consequences for Galactic dynamics. However, our favoredscenario is that of a white dwarf with a rather exotic Fe core, having acooling time compatible with the Hyades age. This is a tantalizingresult that would have implications for the thermonuclear explosion ofwhite dwarfs and explosion theories of degenerate nuclei.Based on observations obtained at: Calar Alto Observatory,Almería, Spain and McDonald Observatory, Texas, USA.
| A Comparative Study of Optical and Ultraviolet Effective Temperatures for DA White Dwarfs from the IUE Archive
We present a comparative study of effective temperatures determined fromthe hydrogen Balmer lines and from the UV energy distribution for 140 DAwhite dwarfs drawn from the International Ultraviolet Explorer (IUE)archive. Our results indicate that the optical and UV temperatures ofthe majority of stars below Teff~40,000 K and within ~75 pcare in fairly good agreement given the uncertainties. At highertemperatures and/or larger distances, however, significant discrepanciesare observed. Several mechanisms are investigated to account for thesediscrepancies, including the effect of interstellar reddening, thepresence of metals in the photosphere, and the existence of unresolvedbinary white dwarfs. The results of our analysis reveal thatwavelength-dependent extinction is the most natural explanation for theobserved temperature differences. We also attempt to predict thedifferences in optical and UV temperatures expected from unresolveddegenerate binaries by performing an exhaustive simulation of compositemodel spectra. In light of these simulations, we then discuss some knowndouble degenerates and identify new binary candidates by restricting ouranalysis to stars located within 75 pc, where the effect of interstellarreddening is significantly reduced.
| Albus 1: A Very Bright White Dwarf Candidate
We have serendipitously discovered a previously unknown, bright source(BT=11.75+/-0.07 mag) with a very blueVT-Ks color, which we have named Albus 1. Aphotometric and astrometric study using Virtual Observatory tools hasshown that it possesses an appreciable proper motion and magnitudes andcolors very similar to those of the well-known white dwarf G191-B2B. Weconsider Albus 1 as a DA-type white dwarf located at about 40 pc. If itsnature is confirmed, Albus 1 would be the sixth brightest isolated whitedwarf in the sky, which would make it an excellent spectrophotometricstandard.
| A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with theSpitzer Space Telescope. Objects were observed simultaneously at 4.5 and8.0 ?m with sensitivities better than 0.1 mJy. This data set can beused to test models of white dwarf atmospheres in a new wavelengthregime, as well as to search for planetary companions and debris disks.
| The mass and luminosity functions and the formation rate of DA white dwarfs in the Sloan Digital Sky Survey
Aims.The SDSS Data Release 1 includes 1833 DA white dwarfs (WDs) andforms the largest homogeneous sample of WDs. This sample provides thebest opportunity to study the statistical properties of WDs. Methods:We adopt a recently established theoretical model to calculate the massand distance of each WD using the observational data. Then we adopt abin-correction method to correct for selection effects and use the 1/Vweight-factor method to calculate the luminosity function, thecontinuous mass function and the formation rate of these WDs. Results:The SDSS DA WD sample is incomplete and suffers seriously from selectioneffects. After corrections for the selection effects, only 531 WDsremain. From this final sample we derive the most up-to-date luminosityfunction and mass function, in which we find a broad peak of WD massescentered around 0.58 Mȯ. The DA WD space density iscalculated as 8.81×10-5 pc-3 and theformation rate is 2.579×10-13 pc-3yr-1. Conclusions: The statistical properties of the SDSS DAWD sample are generally in good agreement with previous observationaland theoretical studies, and provide us information on the formation andevolution of WDs. However, a larger and more complete all-sky WD sampleis still needed to explain some subtle disagreements and unresolvedissues.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/627
| A Search for Kilogauss Magnetic Fields in White Dwarfs and Hot Subdwarf Stars
We present new results of a survey for weak magnetic fields among DAwhite dwarfs, including some brighter hot subdwarf stars. We havedetected variable circular polarization in the H? line of the hotsubdwarf star Feige 34 (spectroscopic type: sdO). From these data, weestimate that the longitudinal magnetic field of this star varies from-1.1+/-3.2 to +9.6+/-2.6 kG, with a mean of about +5 kG and a periodlonger than 2 hr. In this study, we also confirm the magnetic nature ofwhite dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado andcoworkers, and present upper limits of kilogauss longitudinal magneticfields of the five brightest DA white dwarfs. Our data support thefinding of Aznar Cuadrado and coworkers that ~25% of white dwarfs havekilogauss magnetic fields. This frequency also confirms results of earlyestimates obtained using the magnetic field function of white dwarfs(Fabrika & Valyavin).
| Calibration of Synthetic Photometry Using DA White Dwarfs
We have calibrated four major ground-based photometric systems withrespect to the Hubble Space Telescope (HST) absolute flux scale, whichis defined by Vega and four fundamental DA white dwarfs. Thesephotometric systems include the Johnson-Kron-Cousins UBVRI, theStrömgren uvby filters, the Two Micron All Sky SurveyJHKs, and the Sloan Digital Sky Survey ugriz filters.Synthetic magnitudes are calculated from model white dwarf spectrafolded through the published filter response functions; these magnitudesin turn are absolutely calibrated with respect to the HST flux scale.Effective zero-magnitude fluxes and zero-point offsets of each systemare determined. In order to verify the external observationalconsistency, as well as to demonstrate the applicability of thesedefinitions, the synthetic magnitudes are compared with the respectiveobserved magnitudes of larger sets of DA white dwarfs that havewell-determined effective temperatures and surface gravities and span awide range in both of these parameters.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Low-Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey forlow-mass stellar and substellar companions to white dwarfs. A wide-fieldproper-motion survey of 261 white dwarfs was capable of directlydetecting companions at orbital separations between ~100 and 5000 AUwith masses as low as 0.05 Msolar, while a deep near-fieldsearch of 86 white dwarfs was capable of directly detecting companionsat separations between ~50 and 1100 AU with masses as low as 0.02Msolar. Additionally, all white dwarf targets were examinedfor near-infrared excess emission, a technique capable of detectingcompanions at arbitrarily close separations down to masses of 0.05Msolar. No brown dwarf candidates were detected, whichimplies a brown dwarf companion fraction of <0.5% for white dwarfs.In contrast, the stellar companion fraction of white dwarfs as measuredby this survey is 22%, uncorrected for bias. Moreover, most of the knownand suspected stellar companions to white dwarfs are low-mass starswhose masses are only slightly greater than the masses of brown dwarfs.Twenty previously unknown stellar companions were detected, five ofwhich are confirmed or likely white dwarfs themselves, while 15 areconfirmed or likely low-mass stars. Similar to the distribution of coolfield dwarfs as a function of spectral type, the number of coolunevolved dwarf companions peaks at mid-M type. Based on the presentwork, relative to this peak, field L dwarfs appear to be roughly 2-3times more abundant than companion L dwarfs. Additionally, there is noevidence that the initial companion masses have been altered bypost-main-sequence binary interactions.
| Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main-Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC5904) based on images taken with the Hubble Space Telescope. Theresulting color-magnitude diagram reaches below V~27 mag, revealing theupper 2-3 mag of the white dwarf cooling sequence and main-sequencestars 8 mag and more below the turnoff. We fit the main sequence tosubdwarfs of known parallax to obtain a true distance modulus of(m-M)0=14.45+/-0.11 mag. A second distance estimate based onfitting the cluster white dwarf sequence to field white dwarfs withknown parallax yielded (m-M)0=14.67+/-0.18 mag. We discussthe nature of the difference between the two distance estimates andsuggest approaches for reducing the uncertainty in white dwarf fittingestimates for future studies. We couple our distance estimates withextensive photometry of the cluster's RR Lyrae variables to provide acalibration of the RR Lyrae absolute magnitude yieldingMV(RR)=0.42+/-0.10 mag at [Fe/H]=-1.11 dex. We provideanother luminosity calibration in the form of reddening-free Wasenheitfunctions. Comparison of our calibrations with predictions based onrecent models combining stellar evolution and pulsation theories showsencouraging agreement, and the existing differences may provide usefulfeedback to the models.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| A population model of the solar neighbourhood thin disc white dwarfs
We present a model of the solar neighbourhood (d < 100pc) white dwarf(WD) population and show the resulting effective temperature and massdistribution. Our model parameters, in particular the WD coolingtime-scale, are constrained by the models of Chabrier et al. The localinitial mass function and star formation rate (SFR, per unit volume inthe Galactic plane) of Schröder & Pagel are used for thecreation of the synthetic stars from which the WDs originate, as well asthe above authors' grid of evolutionary tracks. Furthermore, we considerin detail the significant depletion of the older (and cooler) WDs bydilution into the column, as caused by the dynamics of the `thin disc'.To verify our synthetic sample, especially the WD cooling time-scale, westudy the temperature distribution of a small but nearly complete,volume-limited sample of observed WDs, which we characterize by a singleand simple indicator R6300: the number ratio of WDs withTeff < 6300K over those with Teff > 6300K.After determination of the bias owing to a residual incompleteness withcool WDs, we find a corrected value of R6300 of 0.68(+/-0.24). This is in good agreement with our WD population model: itstemperature distribution yields R6300= 0.77.For a spherical volume around the Sun, with d < 100pc, our populationmodel suggests a total of about 13700 WDs (omiting part of the WDs witha binary system origin, an estimated 7 +/- 2 per cent of the totalcount). A subsample, limited to magnitude 19.0 (as expected for theEuropean Space Agency mission Gaia), would contain about 7750 WDs ofthis population model. With a less deep magnitude limit of B= 16.0, moretypical of current observed WD samples (SN Ia Progenitor surveY; SPY),the number of objects is reduced to only ~1350. We use this specificsynthetic subsample to test the completeness of the prospective SPY WDsample, which (with over 50 per cent of the candidates now observed) isalready the largest sample of WDs with high-resolution spectra. We findthat within d < 100pc and for B < 16.0 SPY will deliver a fairlycomplete (almost 80 per cent) sample.
| Models of Strange Stars with a Crust and Strange Dwarfs
Strange quark stars with a crust and strange dwarfs consisting of acompact strange quark core and an extended crust are investigated interms of a bag model. The crust, which consists of atomic nuclei anddegenerate electrons, has a limiting density of cr=drip=4.3·1011g/cm3. A series of configurations are calculated for two sets of bagmodel parameters and three different values of cr (109 g/cm3 ≤ρcr ≤ ρdrip) to find the dependence of a star's mass M andradius R on the central density. Sequences of stars ranging from compactstrange stars to extended strange dwarfs are constructed out of strangequark matter with a crust. The effect of the bag model parameters andlimiting crust density ρcr on the parameters of the strange starsand strange dwarfs is examined. The strange dwarfs are compared withordinary white dwarfs and observational differences between the two arepointed out.
| Theoretical UBVRI colors of iron core white dwarfs
We explore photometric properties of hypothetical iron core white dwarfsand compute their expected colors in UBVRI Johnson broadband system.Atmospheres of iron core WDs in this paper consist of pure iron coveredby a pure hydrogen layer of an arbitrary column mass. LTE modelatmospheres and theoretical spectra are calculated on the basis of LosAlamos TOPS opacities and the equation of state from the OPAL project,suitable for nonideal Fe and H gases. We have also computed UBVRI colorsof the models and determined an area on the B-V vs. U-B and B-V vs. V-Iplanes, occupied by both pure Fe, and pure H model atmospheres of WDstars. Finally, we search for iron core white dwarf candidates in theavailable literature.
| Far Ultraviolet Spectroscopic Explorer Observation of the Ultramassive White Dwarf PG 1658+441
We present an analysis of the Far Ultraviolet Spectroscopic Explorer(FUSE) spectrum of the ultramassive (M=1.31 Msolar), magnetic(Bs=2.3 MG) white dwarf PG 1658+441. The far-ultraviolet(FUV) spectrum exhibits very broad Lyman lines and quasi-molecularLy? satellites, but weak Ly? satellites may also be present.PG 1658+441 is the hottest white dwarf known to show these satellitefeatures. We fit the Lyman lines with stellar models and obtainatmospheric parameters consistent with a published analysis of theBalmer lines. By averaging results obtained for the different FUSEsegments, we determine Teff=29,620+/-500 K andlogg=9.31+/-0.07. The models match the data over large portions of thespectrum, but discrepancies remain near the satellite features. Finally,no trace elements have been identified in the FUV spectrum, and weprovide abundance upper limits for C, N, Si, and P.
| An Archive of IUE Low-Dispersion Spectra of the White Dwarf Stars
We have produced an archive of the ultraviolet low-dispersion spectrafor the full set of white dwarf stars observed with the InternationalUltraviolet Explorer (IUE) satellite over the course of its 18 yrmission. This archive contains the spectra of 322 individual degeneratestars which have been processed to optimize the signal-to-noise for eachstar. In particular, all spectra have been corrected for residualtemporal and thermal effects and placed on the Hubble Space TelescopeFaint Object Spectrograph absolute flux scale using procedures describedby Massa & Fitzpatrick. Wherever possible, multiple observations ofindividual stars have been co-added to further enhance signal-to-noiseand have been combined into a single spectrum including the full 1150 to3150 Å wavelength region observed by IUE. The contents of thisspectral archive are described and the details of data reductionprocedures are provided, along with the url for access to the electronicfiles of the processed spectra.
| Modeling of the Lyman gamma satellites in FUSE spectra of DA white dwarfs
We present new theoretical calculations of the line profile of Lymangamma that include transitions in which a photon is absorbed by aneutral hydrogen atom while it interacts with a proton. Models show thattwo absorption features located near 992 Å and 996 Å are dueto H-H+ collisions. These quasi-molecular satellites aresimilar to those that were identified in the wings of Lyman alpha andLyman beta lines of hydrogen-rich white dwarfs. We compute syntheticspectra that take account of these new theoretical profiles and comparethem to the spectra of four DA white dwarfs that were observed withFUSE. The models predict the absorption features that are observed inthe wing of Lyman gamma near 995 Å, and confirm that thesefeatures are quasi-molecular satellites.This paper is dedicated in memory of J. L. Greenstein, discoverer of thequasi-molecular lines in white dwarfs, who passed away on October 21,2002.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| A Determination of the Local Density of White Dwarf Stars
The most recent version of the Catalog of Spectroscopically IdentifiedWhite Dwarfs lists 2249 white dwarf stars. Among these stars are 109white dwarfs that have either reliable trigonometric parallaxes orcolor-based distance moduli that place them at a distance within 20 pcof the Sun. Most of these nearby white dwarfs are isolated stars, but 28(25% of the sample) are in binary systems, including such well-knownsystems as Sirius A/B and Procyon A/B. There are also three doubledegenerate systems in this sample of the local white dwarf population.The sample of local white dwarfs is largely complete out to 13 pc, andthe local density of white dwarf stars is found to be5.0+/-0.7×10-3 pc-3, with a correspondingmass density of 3.4+/-0.5×10-3Msolarpc-3.
| Imaging planets around nearby white dwarfs
We suggest that Jovian planets will survive the late stages of stellarevolution, and that white dwarfs will retain planetary systems in wideorbits (>~5au). Utilizing evolutionary models for Jovian planets, weshow that infrared imaging with 8-m class telescopes of suitable nearbywhite dwarfs should allow us to resolve and detect companions >~3MJUP . Detection of massive planetary companions to nearbywhite dwarfs would prove that such objects can survive the final stagesof stellar evolution, place constraints on the frequency ofmain-sequence stars with planetary systems dynamically similar to ourown and allow direct spectroscopic investigation of their compositionand structure.
| Procyon B: Outside the Iron Box
Procyon B is one of a select group of white dwarf stars in visualbinaries that are used to put stringent constraints on the fundamentalphysics of stellar degeneracy. We present Space Telescope ImagingSpectrograph observations of this elusive white dwarf. Our spectra cover1800-10000 Å and contain carbon Swan bands, Mg II, and numerousiron features. Procyon B is therefore a rare DQZ white dwarf. Ouranalysis of the spectrum yields an effective temperature of 7740+/-50 Kand a radius of 0.01234+/-0.00032 Rsolar. We discuss theimplications of these results on observational support for the whitedwarf mass-radius relation.
| Astrophysics in 2000
It was a year in which some topics selected themselves as importantthrough the sheer numbers of papers published. These include theconnection(s) between galaxies with active central engines and galaxieswith starbursts, the transition from asymptotic giant branch stars towhite dwarfs, gamma-ray bursters, solar data from three major satellitemissions, and the cosmological parameters, including dark matter andvery large scale structure. Several sections are oriented aroundprocesses-accretion, collimation, mergers, and disruptions-shared by anumber of kinds of stars and galaxies. And, of course, there are theusual frivolities of errors, omissions, exceptions, and inventories.
| The evolution of iron white dwarf stars
Recent measurements by Hipparcos provide strong observational evidencesupporting the existence of white dwarf stars with iron-rich corecomposition. Here we examine the evolution of iron-rich white dwarfs,for which the cooling is substancially accelerated as compared with thestandard carbon-oxigen white dwarfs.
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| Takýmyýldýz: | Arabaci |
| Sað Açýklýk: | 06h47m37.99s |
| Yükselim: | +37°30'56.9" |
| Görünürdeki Parlaklýk: | 12.27 |
| Uzaklýk: | 15.406 parsek |
| özdevim Sað Açýklýk: | -230.4 |
| özdevim Yükselim: | -945.1 |
| B-T magnitude: | 11.959 |
| V-T magnitude: | 12.245 |
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